1. Field of the Invention
The present invention relates to an improved automobile air conditioning system. More particularly, the present invention relates to a refrigerant circuit having a slant plate type compressor with a variable displacement mechanism suitable for use in an automobile air conditioning system.
2. Description of the Prior Art
One construction of a slant plate type compressor, particularly a wobble plate type compressor, with a variable capacity mechanism which is suitable for use in an automobile air conditioning system is disclosed in U.S. Pat. No. 3,861,829 issued to Roberts et al. The Roberts et al. '829 patent discloses a wobble plate type compressor which has a cam rotor driving device to drive a plurality of pistons. The slant or incline angle of the slant surface of the wobble plate is varied to change the stroke length of the pistons which changes the displacement of the compressor. Changing the incline angle of the wobble plate is effected by changing the pressure difference between the suction chamber and the crank chamber in which the driving device is located.
In the compressor of the '829 patent, the slant angle of the slant surface is controlled by the pressure in the crank chamber. Typically this control occurs in the following manner. The crank chamber communicates with the suction chamber through an aperture and the opening and closing of the aperture is controlled by a valve mechanism. The valve mechanism generally includes a bellows element and a needle valve, and is located in the suction chamber so that the bellows element operates in accordance with changes in the suction chamber pressure. The pressure of the suction chamber is compared with a predetermined value by the valve mechanism. However, when the predetermined value is below a critical value, there is a possibility of frost forming on the evaporator in the refrigerant circuit. Thus, the predetermined value is usually set higher than the critical value to prevent frost from forming on the evaporator.
Since suction pressure above the critical value is higher than the pressure in the suction chamber when the compressor operates at maximum capacity, the cooling characteristics of the compressor are inferior to those of the same compressor without a variable displacement mechanism. As shown in FIG. 1, the temperature of the air leaving the evaporator cannot fall to the temperature of the air leaving the evaporator when the compressor operates at maximum capacity. In FIG. 1, T2 is the temperature corresponding to the critical value, for example, 4 degrees centigrade. T1 is the temperature when the compressor operates at maximum capacity, for example, 2 degrees centigrade. Accordingly, one of the disadvantages of an automobile air conditioning system including the compressor of the '829 patent is that inner surfaces of the automobile windows are not rapidly demisted when required because the cooling characteristics of the compressor are inferior to those of the same compressor without a variable displacement mechanism.
Roberts et al. '829 discloses a capacity adjusting mechanism used in a wobble plate type compressor. As is typical in this type of compressor, the wobble plate is disposed at a slant or incline angle relative to the drive shaft axis. The wobble plate nutates but does not rotate as the drive shaft rotates to drive the pistons. Capacity adjustment is accomplished by using selective fluid communication between the crank chamber and the suction chamber. This type of capacity adjustment can be used in any type of compressor which uses a slanted plate or surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604 issued to Terauchi discloses this type of capacity adjusting mechanism in a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble plate only nutates, the swash plate both nutates and rotates. The term slant plate type compressor is used herein to include wobble and swash plate type compressors which use a slanted plate or surface in the drive mechanism.
An improved capacity adjusting mechanism is disclosed in U.S. Pat. No. 4,778,348 issued to Kikuchi et al. In the '348 patent, a single controlled compressor solenoid valve is used in combination with a pressure actuated bellows valve (the first valve control device) to improve cooling characteristics and temperature control in the passenger compartment. During the "cool down" stage of an air conditioning system including such a compressor, when the passenger compartment is initially cooled, the second valve control device connects the crank chamber to the suction chamber due to a heat load on the evaporator of the air conditioning system exceeding a predetermined value. Once the heat load drops to the predetermined value, the second valve control device closes the valve. The valve is only reopened if the heat exceeds that predetermined value which will normally occur after the air conditioning system has been turned off and then restarted after a certain time period. Once the second valve control device closes the second valve, the first valve control device solely controls the capacity of the compressor. That is, after the cool down stage, the compressor operates similar to the compressor of the '829 patent. Therefore, the drawbacks of the '829 patent as described above occur during the operation of the automobile air conditioning system disclosed in the '348 patent.
Furthermore, in general, when an automobile air conditioning system is turned on, an "idle up device" is sequentially turned on. The idle up device is used for increasing the number of rotations of an engine in order to compensate for the decrease in the number of rotations of the engine when the compressor is driven during the idling stage of the engine. However, when the temperature of the air out-side the automobile is low, the compressor operates at a controlled displacement because that the heat load on the evaporator is small. This results in a decrease in the driving power supplied to the compressor by the engine. Therefore, under the above conditions, an unnecessary increase in the number of rotations of the engine occurs during the idling stage of the engine due to the operation of the idle up device thereby causing unnecessary fuel consumption.